Synthesis Of Ternary Organic Photovoltaic Materials Based On Thiophene Derivatives For Applications In Solar Cells

Organic semiconductor materials have been widely used in solar cells due to their advantages,such as abundant sources,easy fabrication and low cost.High performance photovoltaic materials play an important role in solar cells for achieving high power conversion efficiency?PCE?.Thiophene derivatives have been used in organic photovoltaic materials due to their advantages of great electron rich and coplanar,thus leading to improved charge transporting properties.Research shows,containing three donor or acceptor units along the backbone chain could adjust the photoelectric property of organic materials effectivily.In this dissertation,some new photovoltaic materials based on thiophene derivatives were designed and synthesized,containing three series of terpolymers and a small mocular hole transporting material.Their photophysical property,thermal stability,electrochemical and charge transport properties were investigated.The photovoltaic performances of resulted materials were further studied in solar cells.The main results are as follows:?1?Two regioregular D-A1-D-A2 type terpolymers PTPDTBT and PDPPTBT are synthesized that incorporate thiophene as donor unit.Owing to its electron deficiency,DPP-based terpolymer PDPPTBT has a broad absorption ranger from 300 to 900 nm,and PTPDTBT based on BT unit exhibits absorption ranging from 300 to 700 nm.The PSC devices are fabricated with the conventional structure,ITO/PEDOT:PSS/polymer:PC₇₁BM/LiF/Al,with the conjugated terpolymers as donor material and PC₇₁BM as acceptor material,respectively.Under AM 1.5G,power of 100 mW cm^–2 simulated sunlight,the PSCs base on PTPDTBT showed a V_oc of 0.90 V,J_sc of 7.88 mA cm^-2,FF of0.41,and PCE of 2.90%.On the same conditions,PDPPTBT based PSC devices obtain a PCE of 1.35%,V_oc of 0.57 V,J_sc of 4.21 mA cm^-2,FF of 0.56.?2?Fluorobenzene and difluorobenzene units are introduced to construct two monomers which have BDT unit as core.Four regioregular D1-A-D2-A type terpolymersPBDTDPP-1FB,PBDTDPP-2FB,PBDTBT-1FBand PBDTBT-2FB are designed and synthesized with DPP and BT groups.PBDTDPP-1FB and PBDTDPP-2FB based on DPP unit have great absorptivity,a wide absorption range in the near-infrared region with onsets above 1000 nm.However,PBDTDPP-1FB and PBDTDPP-2FB are not suitable for application in organic solar cells,mainly due to the deep-lying LUMO levels of two polymers.On the other side,functionalization of the phenylene unit with fluorine atom has a significant effect on the energy levels and charge transportation properties of the polymers.Both PBDTBT-1FB and PBDTBT-2FB exhibit wide-bandgaps of¹.98 eV with deeper-lying HOMO levels.PBDTBT-2FB containing 2FB shows better hole transportation abilities than PBDTBT-1FB.As a result,optimized devices based on polymer PBDTBT-1FB containing 1FB has reached PCE of 2.16%,with V_oc of 0.95 V.The photovoltaic performance of PBDTBT-2FB is improved by increasing its V_oc up to 1.0 V,resulting in a PCE of 3.34%.The wide-bandgap polymers containing fluorinated phenylene units result in the simultaneous promotion of V_oc.?3?A series of D-A1-D-A2 random polymers consisting of TPD and BTO moieties as two acceptor units are synthesized and characterized.Three different donor units?thiophene?T?,thieno[3,2-b]thiophene?TT?and terthiophene?3T??have a marked effect on the optoelectronic properties,solution process ability,charge carrier transport,and the resulting photovoltaic performances.Among the three polymers,the terpolymer PTPDBTO-T containing T group has the deepest HOMO and LUMO level,which resultes in PSCs with the highest V_oc of0.93 V,the devices based on PTPDBTO-T afford PCE of 6.37%,J_scc of 11.05 mA cm^–2,FF of 0.62 with an active layer thickness of 183 nm.The PTPDBTO-TT terpolymer with the TT backbone that has the highest light harvesting and charge carrier transport properties showed the highest J_sc of 12.56 mA cm^–22 in PSCs.A superior PCE of 6.92%with FF of 0.61 and V_oc of 0.90 V has been demonstrated at active layer thickness of 285 nm in devices based on PTPDBTO-TT.Notably,PSCs based on PTPDBTO-T and PTPDBTO-TT could maintain PCEs over 6%and FF over 0.60 at the active layer thickness range of100–285 nm.The device performance of PTPDBTO-T and PTPDBTO-TT are insensitive to the thickness of active layer,suggesting that PTPDBTO-T and PTPDBTO-TT are promising candidate for practical applications via solution printing technology.?4?We develop a novel fluorinated small molecule BDT2FMeDPA,which have BDT unit as core,and further apply it as a dopant-free hole transporting material?HTM?for perovskite solar cells with the vacuum-free carbon cathode.With the introduction of fluorine atoms in BDT2FMeDPA,the hole mobility and conductivity are significantly enhanced with an appropriate energy level alignment as compare with its non-fluorinated analogue?BDT0FMeDPA?.The optimized perovskite solar cells with pristine BDT2FMeDPA afford a decent PCE of 14.5%,which is considerably higher than the devices using its non-fluorinated analogue?11.3%?under equivalent conditions.Moreover,the perovskite solar cells incorporating BDT2FMeDPA also show good long-term stability in air,maintaining over 90%of its initial efficiency after 30 days under ambient atmosphere,due largely to its hydrophobic characteristic.The present work not only emphasizes that the performance of HTM can be effectively tuned through the introduction of fluorine atoms,but also provides insights into future design of dopant-free HTM for low-cost,stable and highly efficient perovskite solar cells.